How to remove borders from existing pdf form fields

Hello all, I've looked through the documentation and have tried different things, but I'm still unable to remove the borders from form fields in an existing pdf.  The following code successfully sets the value of txtName but the border remains:

using (Aspose.Pdf.Document pdfDocument = new Aspose.Pdf.Document(pdfFile))
{
    TextBoxField txtName = pdfDocument.Form["txtName"] as TextBoxField;
    txtName.Value = "name value from aspose";
    txtName.Border.Width = 0;
{

Tried setting it to an empty Border() object and a few other things, with no effect.

Not sure what else to try.  Grateful for any help -